Campylobacter jejuni and Campylobacter coil are the two most commonly isolated species of campylobacter that cause human infection. These organisms cause high rates of gastroenteritis worldwide, with the number of cases often exceeding that for Salmonella, Shigella and Enterotoxigenic E. coli combined (Butzler J P, Clinical Microbiology and Infection 2004). Furthermore, C. jejuni infection has been linked to the development of Guillain-Barré Syndrome, the most common cause of pathogen-caused paralysis since the eradication of polio (for reviews see: Kaida K, Glycobiology, 2009; Bereswill S & Kist M, Current Opinion in Infectious Diseases, 2003). Other Campylobacter species have been recognized as emerging pathogens in human gastroenteritis (C. upsaliensis, C. hyointestinalis) were associated with inflammatory bowel disease in children and with gingivitis, periodontitis, and human abortions (C. retus, C. concisus) (Zhang L S et al., Journal of Clinical Microbiology, 2009) and in causing venereal disease and infertility in livestock (especially cattle; C. fetus venerealis), and sheep abortions (C. fetus fetus) (Butzler J P, Clinical Microbiology and Infection, 2004 and references therein).
Since the publication of the first C. jejuni genome sequence in 2000 (Parkhill J et al., Nature, 2000), several other campylobacter genome sequences have been reported. Unlike the majority of bacteria that have been described to date, all campylobacters contain conserved pgl genes required for N-linked protein glycosylation (Szymanski C M & Wren B W, Nature Reviews Microbiology 2005; Nothaft H & Szymanski C M, Nature Reviews Microbiology, 2010).
In eukaryotes, glycosylated proteins are ubiquitous components of extracellular matrices and cellular surfaces. Their oligosaccharide moieties are implicated in a wide range of cell-cell and cell-matrix recognition events that are vital in biological processes ranging from immune recognition to cancer development. Glycosylation was previously considered to be restricted to eukaryotes, however through advances in analytical methods and genome sequencing, there have been increasing reports of both 0-linked and N-linked protein glycosylation pathways in bacteria (Nothaft H & Szymanski C M, Nature Reviews Microbiology, 2010). Since the discovery of the first general protein glycosylation pathway in bacteria (Szymanski C M et al., Molecular Microbiology 1999), the demonstration that the C. jejuni glycans are attached through an N-linkage en bloc (Kelly J H et al., Journal of Bacteriology 2006, Wacker M et al., Science 2002, Young N M et al., Journal of Biological Chemistry, 2002) and that the pathway not only can be functionally transferred into Escherichia coli (Wacker M et al., Science, 2002), but that the oligosaccharyltransferase enzyme (PglB) is capable of adding foreign sugars to protein (Feldman M et al., PNAS 2005), a surge of research activities has resulted in further characterization and exploitation of this system.
The detailed structure of the unique C. jejuni N-linked heptasaccharide has been described (Young N M et al., Journal of Biological Chemistry, 2002). Using methods such as high resolution magic angle spinning (HR-MAS) NMR (Szymanski C M et al., Journal of Biological Chemistry, 2003), it has been shown that this heptasaccharide is conserved in structure in both C. jejuni and C. coli. 
An intermediate in the C. jejuni N-linked glycosylation pathway has been described, namely a free (oligo-) heptasaccharide (fOS)—a soluble component of the C. jejuni periplasmic space (Liu X et al., Analytical Chemistry, 2006). This fOS has the identical structure as the N-linked oligosaccharide added onto proteins (Nothaft H et al., PNAS 2009). Under laboratory growth conditions, the ratio of fOS versus heptasaccharide N-linked to protein is approximately 9:1. The fOS in C. jejuni plays a role in osmoregulation similar to bacterial periplasmic glucans and this pathway can be manipulated by altering the environmental osmolyte concentration (Nothaft H et al., PNAS 2009).
FIG. 1 shows N-linked protein glycosylation and free oligosaccharides in C. jejuni. The undecaprenyl-pyrophosphate-linked heptasaccharide is assembled in the cytosol by the addition of nucleotide activated sugars (Szymanski C M et al., Journal of Biological Chemistry, 2003; Szymanski C M et al., Trends Microbiology 2003). The complete heptasaccharide is translocated across the inner membrane to the periplasm by the ABC transporter PglK (Alaimo C et al., EMBO Journal, 2006). The oligosaccharide is transferred to the amino group of asparagine in the protein consensus sequence, D/E-X1-N-X2-S/T, wherein X1, X2 can be any amino acid except proline, by PglB (Kowarik M et al., EMBO Journal 2006; Young N M et al., Journal of Biological Chemistry, 2002). In addition, large amounts of free heptasaccharide (fOS) can be found in C. jejuni (Liu X et al., Analytical Chemistry, 2006); the fOS to N-glycan ratio was determined to be 9:1. GlcNAc, N-acetylgalactosamine; Bacillosamine (Bac), 2,4-diacetamido-2,4,6-trideoxyglucose; GalNAc, N-acetylgalactosamine; Glc, Glucose (adapted from Szymanski C M et al., Trends Microbiology, 2003).